Lower bound of the (possibly infinite) radius of the universe, if it is a 3-sphere, according to one estimate using the WMAP data at 95% confidence.[45] It equivalently implies that there are at minimum 21 particle horizon-sized volumes in the universe.

1.6 × 10−11yoctometres (1.6 × 10−35 metres) – the Planck length (Measures of distance shorter than this are considered nonsensical and do not make any physical sense, according to current theories of physics).

1 ym – 1 yoctometre, the smallest named subdivision of the metre in the SIbase unit of length, one septillionth of a metre.

Comparison of sizes of semiconductor manufacturing process nodes with some microscopic objects and visible light wavelengths. At this scale, the width of a human hair is about 10 times that of the image.[54]

15 metres – approximate distance the tropical circles of latitude are moving towards the equator and the polar circles are moving towards the poles each year due to a natural, gradual decrease in the Earth's axial tilt

A length of 100 kilometres (about 62 miles), as a rough amount, is relatively common in measurements on Earth and for some astronomical objects.
It is the altitude at which the FAI defines spaceflight to begin.
To help compare orders of magnitude, this section lists lengths between 100 and 1,000 kilometres (105 and 106metres).

Small planets, the Moon and dwarf planets in our solar system have diameters from one to ten million metres. Top row: Mars (left), Mercury (right); bottom row: Moon (left), Pluto (center), and Haumea (right), to scale.

Planets from Venus up to Uranus have diameters from ten to one hundred million metres. Top row: Uranus (left), Neptune (right); middle row: Earth (left), Sirius B (center), and Venus (right), to scale.

Rigel and Aldebaran (top left and right) compared to smaller stars, the Sun (very small dot in lower middle, with orbit of Mercury as yellow ellipse) and transparent sphere with radius of one light minute.

Comparison of size of the Kuiper belt (large faint torus) with the star VY Canis Majoris (within Saturn's orbit), Betelgeuse (inside Jupiter's orbit) and R Doradus (small central red sphere) together with the orbits of Neptune and Uranus, to scale. The yellow ellipses represent the orbits of each planet and the dwarf planet Pluto.

Sedna's orbit (left) is longer than 100 Tm, but other lengths are between 10 and 100 Tm: Comet Hale-Bopp's orbit (lower, faint orange); one light-day (yellow spherical shell with yellow Vernal point arrow as radius); the heliosphere's termination shock (blue shell); and other arrows show positions of Voyager 1 (red) and Pioneer 10 (green). Click on image for larger view and links to other scales.

Lengths with order of magnitude 1e18m: thousand light year radius circle with yellow arrow and 100 light year circle at right with globular cluster Messier 5 within and Carina Nebula in front; globular cluster Omega Centauri to left of both; part of the 1400 light year wide Tarantula Nebula fills the background.

This list includes distances between 1 and 10 exametres (1018 m). To help compare different distances this section lists lengths between 1018m (1 Em or 105.7 light years) and 1019 m (1057 light years).

^ ab10115 is 1 followed by 115 zeroes, or a googol multiplied by a quadrillion. 1010115 is 1 followed by a quadrillion googol zeroes. 101010122is 1 followed by 1010122 (a googolplex10 sextillion) zeroes.

^gravitational waves that originate tens of millions of light years from Earth are expected to distort the 4 kilometre mirror spacing by about 10−18 m, less than one-thousandth the charge diameter of a proton. Equivalently, this is a relative change in distance of approximately one part in 1021.
"On 14 September 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10−21."
B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), "Observation of Gravitational Waves from a Binary Black Hole Merger", Phys. Rev. Lett. 116, 061102, published 11 February 2016.

^ abRoberts, Richard W. (1975-06-01). Metric System of Weights and Measures - Guidelines for Use. USA: Director of the National Bureau of Standards. Federal Register FR Doc.75-15798 (1975-06-18). Accordingly, the following units and terms listed in the table of metric units in section 2 of the act of 28 July 1866, that legalized the metric system of weights and measures in the United States, are no longer accepted for use in the United States: myriameter, stere, millier or tonneau, quintal, myriagram, kilo (for kilogram).

1.
Metre
–
The metre or meter, is the base unit of length in the International System of Units. The metre is defined as the length of the path travelled by light in a vacuum in 1/299792458 seconds, the metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole. In 1799, it was redefined in terms of a metre bar. In 1960, the metre was redefined in terms of a number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted, the imperial inch is defined as 0.0254 metres. One metre is about 3 3⁄8 inches longer than a yard, Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations except the United States and the Philippines, which use meter. Measuring devices are spelled -meter in all variants of English, the suffix -meter has the same Greek origin as the unit of length. This range of uses is found in Latin, French, English. Thus calls for measurement and moderation. In 1668 the English cleric and philosopher John Wilkins proposed in an essay a decimal-based unit of length, as a result of the French Revolution, the French Academy of Sciences charged a commission with determining a single scale for all measures. In 1668, Wilkins proposed using Christopher Wrens suggestion of defining the metre using a pendulum with a length which produced a half-period of one second, christiaan Huygens had observed that length to be 38 Rijnland inches or 39.26 English inches. This is the equivalent of what is now known to be 997 mm, no official action was taken regarding this suggestion. In the 18th century, there were two approaches to the definition of the unit of length. One favoured Wilkins approach, to define the metre in terms of the length of a pendulum which produced a half-period of one second. The other approach was to define the metre as one ten-millionth of the length of a quadrant along the Earths meridian, that is, the distance from the Equator to the North Pole. This means that the quadrant would have defined as exactly 10000000 metres at that time. To establish a universally accepted foundation for the definition of the metre, more measurements of this meridian were needed. This portion of the meridian, assumed to be the length as the Paris meridian, was to serve as the basis for the length of the half meridian connecting the North Pole with the Equator

2.
Electron
–
The electron is a subatomic particle, symbol e− or β−, with a negative elementary electric charge. Electrons belong to the first generation of the lepton particle family, the electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the include a intrinsic angular momentum of a half-integer value, expressed in units of the reduced Planck constant. As it is a fermion, no two electrons can occupy the same state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of particles and waves, they can collide with other particles and can be diffracted like light. Since an electron has charge, it has an electric field. Electromagnetic fields produced from other sources will affect the motion of an electron according to the Lorentz force law, electrons radiate or absorb energy in the form of photons when they are accelerated. Laboratory instruments are capable of trapping individual electrons as well as electron plasma by the use of electromagnetic fields, special telescopes can detect electron plasma in outer space. Electrons are involved in applications such as electronics, welding, cathode ray tubes, electron microscopes, radiation therapy, lasers, gaseous ionization detectors. Interactions involving electrons with other particles are of interest in fields such as chemistry. The Coulomb force interaction between the positive protons within atomic nuclei and the negative electrons without, allows the composition of the two known as atoms, ionization or differences in the proportions of negative electrons versus positive nuclei changes the binding energy of an atomic system. The exchange or sharing of the electrons between two or more atoms is the cause of chemical bonding. In 1838, British natural philosopher Richard Laming first hypothesized the concept of a quantity of electric charge to explain the chemical properties of atoms. Irish physicist George Johnstone Stoney named this charge electron in 1891, electrons can also participate in nuclear reactions, such as nucleosynthesis in stars, where they are known as beta particles. Electrons can be created through beta decay of isotopes and in high-energy collisions. The antiparticle of the electron is called the positron, it is identical to the electron except that it carries electrical, when an electron collides with a positron, both particles can be totally annihilated, producing gamma ray photons. The ancient Greeks noticed that amber attracted small objects when rubbed with fur, along with lightning, this phenomenon is one of humanitys earliest recorded experiences with electricity. In his 1600 treatise De Magnete, the English scientist William Gilbert coined the New Latin term electricus, both electric and electricity are derived from the Latin ēlectrum, which came from the Greek word for amber, ἤλεκτρον

3.
Wavelength
–
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the waves shape repeats, and thus the inverse of the spatial frequency. Wavelength is commonly designated by the Greek letter lambda, the concept can also be applied to periodic waves of non-sinusoidal shape. The term wavelength is also applied to modulated waves. Wavelength depends on the medium that a wave travels through, examples of wave-like phenomena are sound waves, light, water waves and periodic electrical signals in a conductor. A sound wave is a variation in air pressure, while in light and other electromagnetic radiation the strength of the electric, water waves are variations in the height of a body of water. In a crystal lattice vibration, atomic positions vary, wavelength is a measure of the distance between repetitions of a shape feature such as peaks, valleys, or zero-crossings, not a measure of how far any given particle moves. For example, in waves over deep water a particle near the waters surface moves in a circle of the same diameter as the wave height. The range of wavelengths or frequencies for wave phenomena is called a spectrum, the name originated with the visible light spectrum but now can be applied to the entire electromagnetic spectrum as well as to a sound spectrum or vibration spectrum. In linear media, any pattern can be described in terms of the independent propagation of sinusoidal components. The wavelength λ of a sinusoidal waveform traveling at constant speed v is given by λ = v f, in a dispersive medium, the phase speed itself depends upon the frequency of the wave, making the relationship between wavelength and frequency nonlinear. In the case of electromagnetic radiation—such as light—in free space, the speed is the speed of light. Thus the wavelength of a 100 MHz electromagnetic wave is about, the wavelength of visible light ranges from deep red, roughly 700 nm, to violet, roughly 400 nm. For sound waves in air, the speed of sound is 343 m/s, the wavelengths of sound frequencies audible to the human ear are thus between approximately 17 m and 17 mm, respectively. Note that the wavelengths in audible sound are much longer than those in visible light, a standing wave is an undulatory motion that stays in one place. A sinusoidal standing wave includes stationary points of no motion, called nodes, the upper figure shows three standing waves in a box. The walls of the box are considered to require the wave to have nodes at the walls of the box determining which wavelengths are allowed, the stationary wave can be viewed as the sum of two traveling sinusoidal waves of oppositely directed velocities. Consequently, wavelength, period, and wave velocity are related just as for a traveling wave, for example, the speed of light can be determined from observation of standing waves in a metal box containing an ideal vacuum. In that case, the k, the magnitude of k, is still in the same relationship with wavelength as shown above

4.
Gamma ray
–
Gamma ray, denoted by the lower-case Greek letter gamma, is penetrating electromagnetic radiation of a kind arising from the radioactive decay of atomic nuclei. It consists of photons in the highest observed range of photon energy, paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays, Rutherford had previously discovered two other types of radioactive decay, which he named alpha and beta rays. Gamma rays are able to ionize atoms, and are thus biologically hazardous. The decay of a nucleus from a high energy state to a lower energy state. Natural sources of gamma rays on Earth are observed in the decay of radionuclides. There are rare terrestrial natural sources, such as lightning strikes and terrestrial gamma-ray flashes, However, a large fraction of such astronomical gamma rays are screened by Earths atmosphere and can only be detected by spacecraft. Gamma rays typically have frequencies above 10 exahertz, and therefore have energies above 100 keV and wavelengths less than 10 picometers, However, this is not a strict definition, but rather only a rule-of-thumb description for natural processes. Electromagnetic radiation from radioactive decay of nuclei is referred to as gamma rays no matter its energy. This radiation commonly has energy of a few hundred keV, in astronomy, gamma rays are defined by their energy, and no production process needs to be specified. The energies of gamma rays from astronomical sources range to over 10 TeV, a notable example is the extremely powerful bursts of high-energy radiation referred to as long duration gamma-ray bursts, of energies higher than can be produced by radioactive decay. These bursts of gamma rays are thought to be due to the collapse of stars called hypernovae, the first gamma ray source to be discovered historically was the radioactive decay process called gamma decay. In this type of decay, a nucleus emits a gamma ray almost immediately upon formation. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, However, Villard did not consider naming them as a different fundamental type. Rutherford also noted that gamma rays were not deflected by a field, another property making them unlike alpha. Gamma rays were first thought to be particles with mass, like alpha, Rutherford initially believed that they might be extremely fast beta particles, but their failure to be deflected by a magnetic field indicated that they had no charge. In 1914, gamma rays were observed to be reflected from crystal surfaces, Rutherford and his coworker Edward Andrade measured the wavelengths of gamma rays from radium, and found that they were similar to X-rays, but with shorter wavelengths and higher frequency. This was eventually recognized as giving them more energy per photon

5.
X-ray
–
X-radiation is a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz, X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. Spelling of X-ray in the English language includes the variants x-ray, xray, X-rays with high photon energies are called hard X-rays, while those with lower energy are called soft X-rays. Due to their ability, hard X-rays are widely used to image the inside of objects, e. g. in medical radiography. The term X-ray is metonymically used to refer to an image produced using this method. Since the wavelengths of hard X-rays are similar to the size of atoms they are useful for determining crystal structures by X-ray crystallography. By contrast, soft X-rays are easily absorbed in air, the length of 600 eV X-rays in water is less than 1 micrometer. There is no consensus for a definition distinguishing between X-rays and gamma rays, one common practice is to distinguish between the two types of radiation based on their source, X-rays are emitted by electrons, while gamma rays are emitted by the atomic nucleus. This definition has problems, other processes also can generate these high-energy photons. One common alternative is to distinguish X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10−11 m and this criterion assigns a photon to an unambiguous category, but is only possible if wavelength is known. Occasionally, one term or the other is used in specific contexts due to precedent, based on measurement technique. Thus, gamma-rays generated for medical and industrial uses, for radiotherapy, in the ranges of 6–20 MeV. X-ray photons carry enough energy to ionize atoms and disrupt molecular bonds and this makes it a type of ionizing radiation, and therefore harmful to living tissue. A very high radiation dose over a period of time causes radiation sickness. In medical imaging this increased risk is generally greatly outweighed by the benefits of the examination. The ionizing capability of X-rays can be utilized in treatment to kill malignant cells using radiation therapy. It is also used for material characterization using X-ray spectroscopy, hard X-rays can traverse relatively thick objects without being much absorbed or scattered. For this reason, X-rays are widely used to image the inside of visually opaque objects, the most often seen applications are in medical radiography and airport security scanners, but similar techniques are also important in industry and research

6.
Hydrogen
–
Hydrogen is a chemical element with chemical symbol H and atomic number 1. With a standard weight of circa 1.008, hydrogen is the lightest element on the periodic table. Its monatomic form is the most abundant chemical substance in the Universe, non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed protium, has one proton, the universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, since hydrogen readily forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays an important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a charge when it is known as a hydride. The hydrogen cation is written as though composed of a bare proton, Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. Industrial production is mainly from steam reforming natural gas, and less often from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production, mostly for the fertilizer market, Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks. Hydrogen gas is flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol,2 H2 + O2 →2 H2O +572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74%, the explosive reactions may be triggered by spark, heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C, the detection of a burning hydrogen leak may require a flame detector, such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames, the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a mixture of hydrogen to oxygen combined with carbon compounds from the airship skin. H2 reacts with every oxidizing element, the ground state energy level of the electron in a hydrogen atom is −13.6 eV, which is equivalent to an ultraviolet photon of roughly 91 nm wavelength. The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, however, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity. The most complicated treatments allow for the effects of special relativity

7.
Fog
–
Fog consists of visible cloud water droplets or ice crystals suspended in the air at or near the Earths surface. Fog can be considered a type of low-lying cloud and is influenced by nearby bodies of water, topography. In turn, fog has affected many human activities, such as shipping, travel, the term fog is typically distinguished from the more generic term cloud in that fog is low-lying, and the moisture in the fog is often generated locally. By definition, fog reduces visibility to less than 1 kilometre, for aviation purposes in the UK, a visibility of less than 5 kilometres but greater than 999 metres is considered to be mist if the relative humidity is 70% or greater, below 70%, haze is reported. Fog forms when the difference between air temperature and dew point is less than 2.5 °C or 4 °F, Fog begins to form when water vapor condenses into tiny liquid water droplets suspended in the air. Water vapor normally begins to condense on condensation nuclei such as dust, ice, Fog, like its elevated cousin stratus, is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. Fog normally occurs at a relative humidity near 100% and this occurs from either added moisture in the air, or falling ambient air temperature. However, fog can form at lower humidities, and can fail to form with relative humidity at 100%. At 100% relative humidity, the air cannot hold additional moisture, thus, Fog can form suddenly and can dissipate just as rapidly. The sudden formation of fog is known as flash fog, Fog commonly produces precipitation in the form of drizzle or very light snow. Drizzle occurs when the humidity of fog attains 100% and the cloud droplets begin to coalesce into larger droplets. This can occur when the fog layer is lifted and cooled sufficiently, drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point. The inversion boundary varies its altitude primarily in response to the weight of the air above it, the marine layer, and any fogbank it may contain, will be squashed when the pressure is high, and conversely, may expand upwards when the pressure above it is lowering. Fog can form in a number of ways, depending on how the cooling that caused the condensation occurred, radiation fog is formed by the cooling of land after sunset by thermal radiation in calm conditions with clear sky. The warm ground produces condensation in the air by heat conduction. In perfect calm the fog layer can be less than a meter deep, radiation fogs occur at night, and usually do not last long after sunrise, but they can persist all day in the winter months especially in areas bounded by high ground. Radiation fog is most common in autumn and early winter, examples of this phenomenon include the Tule fog. Ground fog is fog that obscures less than 60% of the sky, advection fog occurs when moist air passes over a cool surface by advection and is cooled

8.
Mosquito
–
Mosquitoes are small, midge-like flies that constitute the family Culicidae. Females of most species are ectoparasites, whose tube-like mouthparts pierce the skin to consume blood. The word mosquito is Spanish for little fly, thousands of species feed on the blood of various kinds of hosts, mainly vertebrates, including mammals, birds, reptiles, amphibians, and even some kinds of fish. Some mosquitoes also attack invertebrates, mainly other arthropods, though the loss of blood is seldom of any importance to the victim, the saliva of the mosquito often causes an irritating rash that is a serious nuisance. Much more serious though, are the roles of species of mosquitoes as vectors of diseases. The oldest known mosquito with a similar to modern species was found in 79-million-year-old Canadian amber from the Cretaceous. An older sister species with more primitive features was found in Burmese amber that is 90 to 100 million years old, two mosquito fossils have been found that show very little morphological change in modern mosquitoes against their counterpart from 46 million years ago. These fossils are also the oldest ever found to have blood preserved within their abdomens, the Old and New World Anopheles species are believed to have subsequently diverged about 95 million years ago. The mosquito Anopheles gambiae is currently undergoing speciation into the M and S molecular forms, consequently, some pesticides that work on the M form no longer work on the S form. Over 3,500 species of the Culicidae have already been described and they are generally divided into two subfamilies which in turn comprise some 43 genera. These figures are subject to change, as more species are discovered. The two main subfamilies are the Anophelinae and Culicinae, with their genera as shown in the subsection below, the distinction is of great practical importance because the two subfamilies tend to differ in their significance as vectors of different classes of diseases. Roughly speaking, arboviral diseases such as fever and dengue fever tend to be transmitted by Culicine species. Some transmit various species of malaria, but it is not clear that they ever transmit any form of human malaria. Some species do however transmit various forms of filariasis, much as many Simuliidae do, Anopheline mosquitoes, again not necessarily in the genus Anopheles, sometimes bear pathogenic arboviruses, but it is not yet clear that they ever transmit them as effective vectors. However, all the most important vectors of malaria are Anopheline. Anophelinae Culicinae Mosquitoes are members of a family of nematocerid flies, superficially, mosquitoes resemble crane flies and chironomid flies. In particular, the females of species of mosquitoes are blood-eating pests and dangerous vectors of diseases

9.
Golf ball
–
A golf ball is a special ball designed to be used in the game of golf. Under the rules of golf, a ball has a mass no more than 1.620 oz, has a diameter not less than 1.680 in, and performs within specified velocity, distance. Like golf clubs, golf balls are subject to testing and approval by the R&A and the United States Golf Association, and it is commonly believed that hard wooden, round balls were the first balls used for golf between the 14th through the 17th centuries. It is equally, if not more likely, that leather balls filled with cows hair were used, imported from The Netherlands from at least 1486 onwards, then or later, the featherie ball was developed and introduced. A featherie, or feathery, is a round leather pouch stuffed with chicken or goose feathers and coated with paint. The volume measurement for the feathers was a top hat full of feathers. The feathers were boiled and softened before they were stuffed into the leather pouch, making a featherie was a tedious and time-consuming process. An experienced ball maker could only make a few balls in one day, a single ball would cost between 2 shillings and 5 shillings, which is the equivalent of 10 to 20 US dollars today. There were a few drawbacks to the featherie, first, it was hard to make a perfectly round, spherical ball, and because of this, the featherie often flew irregularly. Also, when the featherie became too wet, its distance would be reduced, despite these, the featherie was still a dramatic improvement over the wooden ball, and remained the standard golf ball well into the 19th century. In 1848, the Rev. Dr. Robert Adams Paterson invented the gutta-percha ball, the guttie was made from dried sap of the Malaysian sapodilla tree. The sap had a feel and could be made round by heating and shaping it in a round mold. Because gutties were cheaper to produce, could be re-formed if they became out-of-round or damaged, accidentally, it was discovered that nicks in the guttie from normal use actually provided a ball with a more consistent ball flight than a guttie with a perfectly smooth surface. Thus, makers began intentionally making indentations into the surface of new balls using either a knife or hammer and chisel, many patterns were tried and used. These new gutties, with protruding nubs left by carving patterned paths across the balls surface, the next breakthrough in golf ball development came in 1898. Coburn Haskell of Cleveland, Ohio had driven to nearby Akron, Ohio for a date with Bertram Work. While he waited in the plant for Work, Haskell picked up some rubber thread, when he bounced the ball, it flew almost to the ceiling. Work suggested Haskell put a cover on the creation, and that was the birth of the 20th century wound golf ball that would replace the guttie bramble ball

10.
Cat
–
The domestic cat is a small, typically furry, carnivorous mammal. They are often called house cats when kept as pets or simply cats when there is no need to distinguish them from other felids. Cats are often valued by humans for companionship and for their ability to hunt vermin, there are more than 70 cat breeds, though different associations proclaim different numbers according to their standards. Cats are similar in anatomy to the felids, with a strong flexible body, quick reflexes, sharp retractable claws. Cat senses fit a crepuscular and predatory ecological niche, cats can hear sounds too faint or too high in frequency for human ears, such as those made by mice and other small animals. They can see in near darkness, like most other mammals, cats have poorer color vision and a better sense of smell than humans. Cats, despite being solitary hunters, are a species and cat communication includes the use of a variety of vocalizations. Cats have a high breeding rate, under controlled breeding, they can be bred and shown as registered pedigree pets, a hobby known as cat fancy. Failure to control the breeding of pet cats by neutering, as well as the abandonment of former household pets, has resulted in numbers of feral cats worldwide. In certain areas outside cats native range, this has contributed, along with destruction and other factors. Cats have been known to extirpate a bird species within specific regions, a genetic study in 2007 concluded that domestic cats are descended from Near Eastern wildcats, having diverged around 8,000 BC in the Middle East. As of a 2007 study, cats are the second most popular pet in the US by number of pets owned, in a 2010 study they were ranked the third most popular pet in the UK, after fish and dogs, with around 8 million being owned. The domestic cat is believed to have evolved from the Near Eastern wildcat, the felids are a rapidly evolving family of mammals that share a common ancestor only 10–15 million years ago and include lions, tigers, cougars and many others. Within this family, domestic cats are part of the genus Felis, members of the genus are found worldwide and include the jungle cat of southeast Asia, European wildcat, African wildcat, the Chinese mountain cat, and the Arabian sand cat, among others. The domestic cat was first classified as Felis catus by Carl Linnaeus in the 10th edition of his Systema Naturae published in 1758, because of modern phylogenetics, domestic cats are usually regarded as another subspecies of the wildcat, F. silvestris. This has resulted in mixed usage of the terms, as the cat can be called by its subspecies name. Wildcats have also referred to as various subspecies of F. catus, but in 2003. The most common name in use for the cat remains F. catus

Closeup of National Prototype Metre Bar No. 27, made in 1889 by the International Bureau of Weights and Measures (BIPM) and given to the United States, which served as the standard for defining all units of length in the US from 1893 to 1960

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's …

Wavelength is decreased in a medium with slower propagation.

A wave on a line of atoms can be interpreted according to a variety of wavelengths.

Pattern of light intensity on a screen for light passing through two slits. The labels on the right refer to the difference of the path lengths from the two slits, which are idealized here as point sources.

A hypernova. Artist's illustration showing the life of a massive star as nuclear fusion converts lighter elements into heavier ones. When fusion no longer generates enough pressure to counteract gravity, the star rapidly collapses to form a black hole. Theoretically, energy may be released during the collapse along the axis of rotation to form a long duration gamma-ray burst.

Taking an X-ray image with early Crookes tube apparatus, late 1800s. The Crookes tube is visible in center. The standing man is viewing his hand with a fluoroscope screen. The seated man is taking a radiograph of his hand by placing it on a photographic plate. No precautions against radiation exposure are taken; its hazards were not known at the time.

1896 plaque published in "Nouvelle Iconographie de la Salpetrière", a medical journal. In the left a hand deformity, in the right same hand seen using radiography. The authors designated the technique as Röntgen photography.

The piano is an acoustic, stringed musical instrument invented in Italy by Bartolomeo Cristofori around the year 1700 …

Image: Grand piano and upright piano

Grand piano by Louis Bas of Villeneuve-lès-Avignon, France, 1781. Earliest French grand piano known to survive; includes an inverted wrestplank and action derived from the work of Bartolomeo Cristofori (ca. 1700) with ornately decorated soundboard.

The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal …

During a total solar eclipse, the solar corona can be seen with the naked eye, during the brief period of totality.

Taken by Hinode's Solar Optical Telescope on 12 January 2007, this image of the Sun reveals the filamentary nature of the plasma connecting regions of different magnetic polarity.

Visible light photograph of sunspot, 13 December 2006

In this false-color ultraviolet image, the Sun shows a C3-class solar flare (white area on upper left), a solar tsunami (wave-like structure, upper right) and multiple filaments of plasma following a magnetic field, rising from the stellar surface.

The ''Galileo'' Probe plunged deep into Jupiter in 1995. It was carried to the Jovian system by the ''Galileo'' spacecraft, where it was released and survived what was then the highest-velocity atmospheric entry yet attempted.

Real image of the northern hemisphere of Saturn, as taken by the Cassini–Huygens mission

The Milky Way arching at a high inclination across the night sky (fish-eye mosaic shot at Paranal, Chile). The bright object is Jupiter in the constellation Sagittarius, and the Magellanic Clouds can be seen on the left. Galactic north is downwards.

Spitzer reveals what cannot be seen in visible light: cooler stars (blue), heated dust (reddish hue), and Sgr A* as bright white spot in the middle.

Newton's color circle, from Opticks of 1704, showing the colors he associated with musical notes. The spectral colors from red to violet are divided by the notes of the musical scale, starting at D. The circle completes a full octave, from D to D. Newton's circle places red, at one end of the spectrum, next to violet, at the other. This reflects the fact that non-spectral purple colors are observed when red and violet light are mixed.

A rendering of the visible spectrum on a gray background produces non-spectral mixtures of pure spectrum with gray, which fit into the sRGB color space.